1. Field of the Invention
The present invention relates to a switching power supply, such as a resonance-type power supply or a half-bridge-type power supply, for example, which outputs a predetermined voltage signal by causing a plurality of switching elements to be alternately turned on and off.
2. Description of the Related Art
In the past, various kinds of switching power supplies have been invented, each of which outputs a predetermined voltage signal by causing a plurality of switching elements to be alternately turned on and off. For example, in a switching power supply utilizing a PWM method of a half-bridge-type converter, a time ratio is adjusted at a constant switching frequency, thereby obtaining a desired output voltage signal. However, in such a switching power supply in which a plurality of switching elements are alternately turned on and off, if there is a time period when a plurality of switching elements are simultaneously turned on, a large short-circuit current flows, and there is a possibility of the power supply being destroyed. Therefore, a dead time is provided in which the plurality of the switching elements are turned off.
Since such a dead time is provided, a switching power supply disclosed in WO2005-076447 includes a first switching element and a second switching element that are alternately turned on and off, and a change of magnetic flux of a transformer due to the turning off of the first switching element is used as a trigger to turn on the second switching element. In addition, the change of magnetic flux of a transformer due to the turning off of the second switching element is used as a trigger to turn on the first switching element. By performing such switching control as described above, the first switching element and the second switching element are prevented from being simultaneously turned on.
However, in the switching power supply disclosed in WO2005-076447, since the on-time of the second switching element is determined by a time constant circuit including a resistor and a capacitor, a switching frequency varies and a switching noise occurs in a wide range in response to the variation of the switching frequency.
In addition, while the time constant circuit is designed taking into account the dead time, since a dead time having the same time length is set in a full load region, namely even in a transient state and a steady state, it is difficult to set an optimal dead time in the steady state in which a dead time that is as long as the dead time in the transient state is not necessary. Accordingly, the above-mentioned switching power supply is less than optimal in terms of efficiency but has a high degree of reliability.